1. Field of the Invention
This invention relates generally to a developer storage and delivery system, and more particularly concerns storing a phase change developer in a developer tank and a process for delivering the phase change developer to a liquid electrophotographic developing system.
2. Background of the Art
In electrophotography, a photoreceptor in the form of a plate, belt, sheet, disk, or drum having an electrically insulating photoconductive element on an electrically conductive substrate is imaged by first uniformly electrostatically charging the surface of the photoconductive element, and then exposing the charged surface to a pattern of light. The light exposure selectively dissipates the charge in the illuminated areas, thereby forming a pattern of charged and uncharged areas (i.e. an electrostatic latent image). A liquid or dry developer is then deposited in either the charged or uncharged areas to create a toned image on the surface of the photoconductive element. The resulting visible image can be fixed to the photoreceptor surface or transferred to a surface of an intermediate transfer material or a suitable receiving medium such as sheets of material, including, for example, paper, transparency, metal, metal coated substrates, composites and the like. The imaging process can be repeated many times on the reusable photoconductive element.
In some electrophotographic imaging systems, the latent images are formed and developed on top of one another in a common or extended imaging region of the photoreceptor. The latent images can also be formed and developed in multiple passes of the photoreceptor around a continuous transport path (i.e., a multi-pass system). Alternatively, the latent images can be formed and developed in a single pass of the photoreceptor around the continuous transport path. A single-pass system enables the multi-color images to be assembled at extremely high speeds relative to the multi-pass system. At each color development station, color developers are applied to the photoreceptor belt, for example, by electrically biased rotating developer rolls.
Image developing methods can be classified into liquid type developing methods and dry type developing methods. The dry type method uses dry developers and the wet type method uses liquid developers.
Dry developers are generally prepared by mixing and dispersing colorant particles and a charge director into a thermoplastic binder resin, followed by milling or micropulverization. The resulted developer particle sizes are generally in the range of about 4 to 10 microns. If the fine powder of a dry developer is scattered, it poses an environmental problem because of its small particle size. Therefore, most dry developers are stored in a cartridge which is easily handled and disposed of Furthermore, the stability of dry developer is usually much better than that of liquid developer.
Liquid developers are usually prepared by dispersing colorant particles, a charge director, and a binder in an insulating liquid (i.e., a carrier or a vehicle). Liquid developer based imaging systems incorporate many features similar to those of dry developer based system. However, liquid developer particles are significantly smaller than dry developer particles. Because of their small particle size, ranging from 3 microns to submicron size, liquid developers are capable of producing very high resolution images. However, liquid developers have some drawbacks.
The major drawbacks of liquid developers are (1) the emission of the liquid carrier from liquid developers to the environment during the drying and transfer process due to inefficient solvent recovery system; (2) the need and difficulty in disposing the waste liquids; (3) the inconvenience of using and handling of liquid developers; (4) and the aggregation and sedimentation instability of materials within liquid developers.
While known liquid developers and processes are suitable for their intended purposes, a need remains for liquid developers and processes that reduce or substantially eliminate the above-mentioned drawbacks. Additionally, there is a need for liquid developers and processes that enable the formation of high quality images on a wide variety of substrates.
There have been attempts to solve some of the above-mentioned drawbacks of liquid developers and dry developers reported in the art. For example, U.S. Pat. No. 5,075,735 to Tsuchiya et al. discloses a developer delivery system comprising stripes or bars of solid developer mounted across a belt. The stripes or bars of solid developer are caused to drop on a heater by a cutter and then melted by the heater into liquid. The resulted liquid developer is then used to develop electrophotographic images.
U.S. Pat. No. 5,783,350 to Matsuoka et al. discloses a meltable developer in a developer tank. The meltable developer is melted by heaters located around the sidewalls of the developer tank and in the bottom of the developer tank. The melted developer is caused to form developed images on a photosensitive body by electrophoresis.
U.S. Pat. No. 5,229,235 to Watanabe et al. discloses an electrophotographic process using a meltable developer. The meltable developer is stored in a developer tank and melted by heaters located in the bottom of the developer tank. The melted developer is caused to form visible images by contacting with electrostatic latent images.
The above attempts still suffer the drawbacks of emission of carrier vapor to the environment; chemical and physical degradation of the developer due to exposure to elevated temperature for long time; and the complexity of the control systems for adjusting the amount and concentration of the molten developer in the developer tank.
This invention provides an improved developer storage and delivery system which eliminates at least some drawbacks of liquid developers and processes while it provides high quality images on a wide variety of substrates.
In a first aspect, the invention features a developer storage and delivery system for liquid electrophotography that includes:
a) a container having an open end;
b) a phase change developer inside said container wherein said phase change developer has a melting point of at least about 22xc2x0 C.; and
c) a heater near said open end wherein said heater melts at least the top surface of said phase change developer.
In a second aspect, the invention features a developer storage and delivery system for liquid electrophotography that includes:
a) a container having an open end;
b) a developer inside said container wherein said developer has a viscosity greater than 10 pascal second at room temperature and pressure (e.g., 18xc2x0 C. and 760 mm Hg); and
c) a heater near said open end wherein said heater lower the viscosity of said developer to less than 0.01 pascal second at room temperature and pressure (e.g., 18xc2x0 C. and 760 mm Hg).
The developer storage and delivery system of the present invention will be described primarily with respect to electrophotographic office printing; however, it is to be understood that these developers are not so limited in their utility and may also be employed in other imaging processes, other printing processes, or other developer transfer processes, such as high speed printing presses, photocopying apparatus, microfilm reproduction devices, facsimile printing, ink jet printer, instrument recording devices, and the like.